1. Field of the Invention
The present invention is broadly concerned with improved, dual mixing shaft preconditioners of the type used upstream of processing devices such as extruders or pellet mills in the production of animal feeds or human foods. More particularly, the invention is concerned with such preconditioners, and processing systems making use thereof, wherein the preconditioners include variable drive mechanisms operably coupled with the mixing shafts and designed to permit selective rotation of the shafts at individual rotational speeds independent of each other.
2. Description of the Prior Art
Preconditioners are widely used in combination with extruders for preparing and blending food materials before further processing and cooking of the same in an extruder. For example, products having a relatively high percentage of flour-like material are often blended with water and treated with steam in a conditioner prior to extrusion. Use of preconditioners is particularly advantageous in preparing pet food or similar products comprising quantities of protein and starch. There are a myriad of pet food formulas in today's market, with widely varying ingredients and quantities thereof. For example, low-calorie pet foods are popular and include very high quantities of starch-bearing materials (e.g., corn and rice). Such low-calorie pet food formulations cannot be subjected to long retention times in a preconditioner, because the starch content thereof tends to become gummy and unsuitable for downstream extrusion processing. On the other hand, standard pet food formulas having far less starch and higher protein contents require long residence times to become properly preconditioned. Therefore, a preconditioner capable of only limited variability of terms of residence times is often not suitable for sophisticated pet food processors.
In recent years there has been an increase in the production of extrusion-processed aquatic feeds used in fish farming. Such aquatic feeds have traditionally included large quantities of fish meal (up to about 70% by weight). However, there is a trend away from using such large quantities of fish meals, owing to the cost and availability of such meal. In lieu thereof processors are using greater quantities of high protein plant ingredients such as soy. A problem with these plant protein sources is that most contain significant quantities of anti-nutritional factors, which must be destroyed during processing. This requires the application of moist heat over a period of time, usually in a preconditioner. Many conventional preconditioners are incapable of fully destroying such anti-nutritional factors, which detracts from their usefulness in the context of modern-day aquatic feeds.
Conventional preconditioning apparatus often includes an elongated vessel having a pair of identical side-by-side, frustocylindrical, intercommunicated mixing chambers each presenting equal areas in transverse cross sections. Each chamber is provided with mixing bars or beaters radially mounted on the rotatable drive shaft aligned with the longitudinal axis of the chamber, and the beaters have a configuration for longitudinally advancing the product from an inlet end of the vessel toward an outlet end of the same as the materials are swept around the frustocylindrical walls. Also, the beaters of each chamber are configured to alternatively pass the product from one chamber to the other when the materials approach the intersection between the chambers.
A series of water inlets are often provided along at least a portion of the length of preconditioning vessels for adding water to the food materials during advancement of the latter longitudinally through the mixing chambers. Obviously, it is highly important that water introduced into preconditioning vessels becomes thoroughly and uniformly blended with materials having a flour-like consistency in order to avoid formation of lumps. Typically, lumps represent a non-homogeneous mixture of the material and water with the material forming the outer surface of the lump receiving the highest percentage of moisture.
Proper blending of water with materials having a flour-like consistency requires both appropriate residence time within the conditioning vessel as well as proper mixing or agitation of the materials with water. As such, increasing the rotational speed of the beaters of conventional preconditioners in an attempt to increase agitation within the vessel causes the materials to pass through the vessel at a greater speed which correspondingly reduces the residence time of the materials within the vessel to values that may be unacceptable. On the other hand, reducing the rotational speed of the beaters to increase residence time within the vessel adversely affects the mixing characteristics of the vessel to the point where proper blending of the materials with water is not achieved. Increasing the overall length of the vessel is not desirable because of mechanical problems associated with the mixing shafts.
Moreover, the structural nature of conventional preconditioning apparatus does not lend itself to flexibility of operation where it is desired, for example, to use one apparatus for processing different materials at varying flow rates. That is, temporarily increasing the length of the apparatus with modular vessel sections in an attempt to increase residence time of materials within the vessel is not a satisfactory solution due to the inherent weight and structural characteristics of the apparatus as well as the predefined material inlets and outlets which are often located at specified positions to pass the materials from one processing stage to the next. As such, it would be desirable to provide a means for varying the residence time of materials passing through a preconditioning apparatus to enable the latter to process different types of materials at optionally varying flow rates.
U.S. Pat. No. 4,752,139 (incorporated by reference herein) describes a class of preconditioners having differently-sized, arcuate mixing chambers with a mixing shaft along the center line of each chamber. The mixing shafts include radially-extending, intercalated mixing elements. In the preconditioners of the '139 patent, the shafts are powered through a single drive motor, using appropriate gearing to maintain a constant speed differential (usually 2:1) between the mixing shafts. These preconditioners are commercialized by Wenger Mfg. Co. of Sabetha, Kans. and have proven to be a significant improvement in the art by increasing system through-puts without corresponding additional operating costs. However, the fixed speed differential design of the preconditioners of the '139 patent can sometimes represent an operational drawback by limiting the range of operational parameters which may otherwise be desirable.